This invention relates generally to cathode ray tube (CRT) and lens assemblies and is particularly directed to a transparent pad optically coupling a CRT and lens assembly in a projection television receiver.
Limitations in image brightness and contrast in standard CRTs have restricted their utility in projection television receivers where a video image is projected onto a reflecting screen. The diffuse reflecting nature of the screen and the increase in size of the image result in substantial attenuation of image brightness and contrast frequently making it difficult to view the projected image at normal room light levels. To improve brightness and contrast many projection television receivers include three CRTs, each providing an image of one of the primary colors of red, green and blue, wherein the three CRTs as well as their respective projected images are maintained in mutual registration. Each CRT is aligned with a respective lens combination by means of which its video image in enlarged and focused upon the televison receiver's projection screen.
Initial projection television receivers included an air gap between the CRT window and the lens assembly. The reflection of light rays from the surface of the CRT's window to the surface of the first lens in the lens assembly, however, gives rise to glare in the thus projected video image and severely limits the contrast ratio in the image projected on the viewing screen. Early attempts to improve the brightness and contrast of the video image air-coupled from the CRT window to the lens assembly involved the use of higher CRT power levels which, unfortunately, created a viewer hazard in the form of excess X-radiation levels.
Attempts to improve video image coupling from the CRT to the lens assembly have made use of solid optical systems which typically include the use of colored beamsplitters near the focal plane of a primary mirror of the optical system, such as disclosed in U.S. Pat. No. 4,027,328 to Lessman, and liquid coupling systems which attempt to match the refractive indices of the CRT and lens assembly and also frequently provide cooling for the CRT. Examples of liquid optical coupling arrangements can be found in U.S. Pat. Nos. 4,151,554 to Tucker (crossed dichroic mirrors disposed within a liquid filled chamber for presenting a single image), 4,511,927 to Bauer (a closed chamber disposed between and in contact with the CRT and lens assembly and including a refractive index-matched liquid), and 4,405,949 to Hockenbrock et al (liquid optical coupling arrangement which also affords CRT cooling), which is assigned to the assignee of the present application. While affording improved video image brightness and contrast, prior art approaches have generally suffered from excessive complexity and expense and somewhat low reliability. For example, the solid optical CRT-lens interface system which includes internal beamsplitters requires a precisely shaped optical arrangement with a complicated internal structure, while many of the liquid optical coupling and cooling arrangements are subject to leakage of the liquid refractive material/coolant.
The above cross-referenced patent application contemplates an optical coupling pad disposed between and in intimate contact with the CRT's window and the lens assembly and includes a multi-layer optical coupling pad having two outer, relatively hard layers, a first intermediate layer having relatively high mechanical tensile strength, and a second intermediate layer which is very soft and provides the pad with high compressibility. The four layers of the optical pad are each comprised of a respective silicone resin in different proportions to a given catalyst. While substantially improving optical coupling between the CRT and lens assembly, this multi-layer arrangement requires a somewhat complicated manufacturing procedure which increases the cost of this approach.
The present invention represents an improvement over the prior art and is directed to a single layer optical coupling pad disposed in tight fitting relation between a CRT and lens assembly in a projection television receiver which not only exhibits excellent optical and mechanical coupling characteristics, but also is inexpensively manufactured, easily installed and removed from in between the CRT and lens assembly, and provides greater CRT protection during installation.